Composite structures, such as ball bats for baseball or softball, may be made with one or more plies of composite laminate material, which typically include a fiber-reinforced matrix material. In a typical composite structure formed with multiple plies of composite laminate material, the proportion of matrix material (sometimes in the form of resin) relative to the proportion of fiber material is higher between the plies (in the interlaminar interfaces) than in the laminate plies themselves. These areas, and other areas in which the matrix material makes up much or all of the assembly relative to traditional reinforcing materials, are typically referred to as “resin-rich” areas. Resin-rich areas tend to be weaker than areas reinforced with more fibers. Designers of composite structures consider these areas when determining the overall strength of the structure. For example, designers often analyze the interlaminar shear strength of an assembled ball bat.
During repeated use of composite structures such as ball bats, the matrix or resin of the composite material tends to crack, and the fibers tend to stretch or break. Sometimes the composite material develops interlaminar failures, which involve plies of the composite materials separating or delaminating from each other along a failure plane between the plies in the interlaminar interface. For example, the plies may separate along the resin rich areas.
In ball bats, this “break-in” reduces stiffness and increases the elasticity or trampoline effect of a bat against a ball, which tends to temporarily increase bat performance. As a bat breaks in, and before it fully fails (for example, before the bat wall experiences a through-thickness failure), it may exceed performance limitations specified by a governing body, such as limitations related to batted ball speed. Some such limitations are specifically aimed at regulating the performance of a bat that has been broken in from normal use, such as BBCOR (“Bat-Ball Coefficient of Restitution”) limitations.
Some unscrupulous baseball or softball players choose to intentionally break in composite bats to increase performance. Intentional break-in processes may be referred to as accelerated break-in (ABI) and may include techniques such as “rolling” a bat or otherwise compressing it, or generating hard hits to the bat with an object other than a ball. Such processes tend to be more abusive than break-in during normal use, and they exploit the relatively weak interlaminar shear strength of resin-rich areas found in the composite structures of typical ball bats to try to increase batted ball speed. Some sports governing bodies require that composite bats meet certain standards even after an ABI procedure in order to limit the increase in performance from use and abuse of a composite bat (this may be referred to as “ABI testing”).
Nanomaterials, which are generally understood to include materials whose individual portions are very small, such as on the order of 1 to 1000 nanometers or other small sizes, are recognized as having unique chemical and structural properties that enhance the properties of ordinary materials. Composite materials may benefit from nanotechnology. For example, some composite material suppliers include nano-additives in the matrix material of a pre-impregnated (“pre-preg”) composite laminate material. Some manufacturers or suppliers may provide nano-additives to the matrix in a resin transfer molding (RTM) process. But existing methods of including nano-additives are inefficient in terms of cost and wasted materials.
Discontinuities in composite plies or the fibers within composite plies, such as ends of composite plies or fibers, create weaknesses in overall composite structures, especially where the ends abut each other or are otherwise joined together. Accordingly, composite plies are generally used in long continuous sections to avoid weaknesses in high-stress zones of the composite structure. In turn, the matrix material must be distributed throughout the entire length of a composite ply. But nano-additives can be expensive or in limited supply. So including nano-additives in all of the matrix material increases production costs or otherwise complicates production. For example, when composite plies are cut or trimmed, nano-additives may be wasted, or unnecessary joints or weaknesses may be created in the assembly. There is a need for efficient enhancement of composite structures using nano-additives.